1. Field of the Invention
This invention relates to the field of analog-to-digital conversion, and in particular to an ultra low power analog-to-digital converter (ADC) for use in applications such as implantable devices where current consumption is a very important parameter.
2. Description of Related Art
In low power applications it is often not possible to afford the luxury of having a continuous ADC running. The analog signals of interest are filtered and amplified, and then compared to a threshold triggering level at which some action is taken, such as turning on the ADC. Unfortunately, the signal shape is often of interest shortly before and immediately after the triggering event. With prior art devices the signal shape before the trigger signal is lost even if the ADC is started immediately. The prior art does not provide a way of determining the shape of the signal prior to triggering of the ADC.
FIG. 1 illustrates a prior art ADC. An analog input signal, such as a heart signal, is fed through a pre-amplifier 10 to amplifier 11 with programmable gain, where it is split into two portions. Amplifier 11 can be programmed to allow for the fact that different people produce heart signals of different amplitude. One portion of the signal is passed to the input of the ADC 12, and another portion is passed to the bandpass filter 13, from where it is fed to comparator 14.
The ADC comprises a digitizing circuit 15 and sampling capacitor 16. The digitizing circuit 15 takes a sampled analog signal and produces a digital output signal in a manner well known in the art. If the filtered signal exceeds a threshold level, comparator 14 generates a trigger signal, in the form of an output pulse going high, that activates ADC 12, which outputs a digital signal at output 17. The input of the ADC 12 is connected to the sampling capacitor 16 through switch 18, which is normally turned off when the ADC is inactive. The sampling capacitor is connected to digitizing circuit 15 through switch 19 and provides a single sample-and-hold circuit that stores the current value of the analog input signal.
After processing, the digitized output signal is stored in RAM for later study by the doctor when the patient goes to hospital. However, since the ADC is not triggered until the threshold is reached, the initial portion of the signal of interest is lost. It is not possible to lower the trigger threshold since then the ADC would be continually activated unnecessarily.